1. Field of the Invention
The present invention relates to a video signal reproducing apparatus, and more particularly to an apparatus for performing slow-motion reproduction of video signals recorded on a tape by driving the tape intermittently.
2. Description of the Prior Art
In a helical scanning type of video recording and reproducing apparatus, the video signals are successively recorded by two rotating heads on tracks oblique to the longitudinal direction of a tape. Usually, the two rotating heads have different azimuth angles for the sake of high density recording, in which adjacent tracks of the tape are abutting or partially overlapping each other. However, the cross-talk signal from the adjacent tracks can be effectively reduced due to azimuth loss of the heads.
For slow-motion reproduction of these recorded video signals, the tape is driven at a speed which is slower than that used in the recording mode (or normal reproducing mode). Accordingly, in reproduction, the rotating heads repeatedly trace the respective tracks a number of times, whereby the reproduced picture has a slower motion than it would have if reproduced in a normal reproducing mode. It is in this way that slow-motion reproduction is achieved. Moreover, when the tape is stopped, a single track is repeatedly traced by the rotating heads to reproduce a still picture.
In the slow-motion reproducing mode or the still picture reproducing mode, the rotating heads rotate at the same speed as in the normal reproducing mode, while the tape is driven at a slower speed than in the normal reproducing mode or is stopped completely. As a result, the inclination angle of the track traced by the rotating head on the tape in the slow-motion and still picture reproducing mode differs from the inclination angle of the track in a recording mode (or normal reproducing mode) and this difference in inclination gives rise to tracking deviation.
In reproduction, when a rotating head having a first azimuth angle deviates from a track to be scanned and crosses another track which is recorded by a different rotating head having a second azimuth angle, a noise bar in the reproduced picture is generated.
In order to overcome the above-described disadvantage, previously known and diclosed an improved system, in U.S. Pat. No. 4,190,869, can be used in which the tape travels or shifts intermittently by rotating a motor driven capstan intermittently. During the time interval that the tape is stopped, the same tracks are scanned a number of times by rotating heads having azimuth angles corresponding to the tracks to be scanned, resulting in the still reproduction. On the other hand, during the time interval that the tape is moved at a normal speed, the normal reproducing mode is carried out by the rotating heads. The slow-motion reproduction is performed by combining the still and normal reproducing modes. It is herein noted that the number of times that the same track is scanned depends on the slow-motion speed.
With the tape being moved intermittently in the above manner by pulse-driving a capstan motor during normal reproduction, it is seen that a rising time of rotation of a capstan is changed in response to a slow-motion ratio 1/N. Consequently a change or deviation in the stop-position of the tape when reproducing in the still mode occurs because the mechanical energies stored in a driving force transmission mechanism, such as a belt interconnected between the capstan flywheel and the capstan motor, are different in response to the slow-motion ratio. Accordingly, the rising time of the capstan becomes shorter, as the slow-motion speed is faster.
As a result, a signal-to-noise ratio of the reproduced slow-motion picture will deteriorate gradually, since such deviations in the stop-positions are cumulative and therefore become greater each time the tape is stopped during the slow-motion reproduction.